Mechanical properties in thermoelectric oxides: Ideal strength, deformation mechanism, and fracture toughness

The recent dramatic improvements in high-performance thermoelectric (TE) oxides provide new exciting applications in the TE field, but the mechanical properties so important for engineering applications remain largely unexplored. Based on density functional theory (DFT) calculations, we report the ideal strength, deformation mechanism, and fracture toughness of such TE oxides as n-type ZnO and SrTiO₃ and p-type BiCuSeO and NaCo₂O₄. The Zn–O and Ti–O bonds forming the 3D Zn–O and Ti–O frameworks dominate the deformation and failure mechanisms of ZnO and SrTiO₃, respectively. Due to the higher stiffness of Ti–O octahedra compared with that of Zn–O tetrahedra, SrTiO₃ exhibits more robust macro-mechanical properties such as elastic modulus and fracture toughness than ZnO. The Bi–Se and Na–O bonds, which couple the different 2D substructures, are responsible for the relative slip in BiCuSeO and NaCo₂O₄, respectively. Since the Zn–O and Ti–O bonds are much stronger than the Bi–Se and Na–O bonds, we find that n-type ZnO and SrTiO₃ have a higher ideal strength and fracture toughness compared with p-type BiCuSeO and NaCo₂O₄. This work reveals that for TE module applications of oxides, it is most important to significantly improve the mechanical properties of the p-leg.